To solve the "Cannot read property 'querySelectorAll' of undefined" error, make sure you're using the correct class name to access th DOM element. The error often occurs after providing an invalid class name to the `getElementsByClassName` method.

Cannot read property 'querySelectorAll' of Undefined in JS

## Cannot read property 'querySelectorAll' of Undefined

**The "Cannot read property 'querySelectorAll' of undefined" error occurs for 2
reasons:**

1. Calling the `querySelectorAll()` method on an `undefined` value (DOM element
   that doesn't exist).
2. Placing the JS script tag above the HTML where the DOM elements are declared.

```javascript:index.js
const el = undefined;

// ⛔️ Cannot read properties of undefined (reading 'querySelectorAll')
el.querySelectorAll('div.box');
```

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**To solve the "Cannot read property 'querySelectorAll' of undefined" error,
make sure you're using the correct class name to access th DOM element. The
error often occurs after providing an invalid class name to the
`getElementsByClassName` method.**

```javascript:index.js
const elements = document.getElementsByClassName('does-not-exist');
console.log(elements); // 👉️ []

// ⛔️ Cannot read properties of undefined (reading 'querySelectorAll')
const boxes = elements[0].querySelectorAll('div.box');
```

We passed a non-existent class name to the `getElementsByClassName()` method and
got an empty array-like object back.

Accessing an array at an index out of bounds returns `undefined`. Calling the
`querySelectorAll()` method on an `undefined` value throws the error.

**To solve the "Cannot read property 'querySelectorAll' of undefined" error,
place the JS script tag at the bottom of the body tag. The script should run
after the DOM elements have been created.**

```html:index.html
<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
  </head>
  <body>
    <div class="container">
      <div class="box">Box 1</div>
      <div class="box">Box 2</div>
    </div>

    
    <script src="index.js"></script>
  </body>
</html>
```

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Notice that we placed the JS script tag at the bottom of the body tag.

Had we placed the script tag above the `div` element, the `index.js` file
wouldn't have access to the `div` elements.

```javascript:index.js
const elements = document.getElementsByClassName('container');
console.log(elements); // 👉️ [div.container]

// ✅ Works
const boxes = elements[0].querySelectorAll('div.box');
console.log(boxes); // 👉️ [div.box, div.box]
```

## Conclusion

The "Cannot read property 'querySelectorAll' of undefined" error occurs when
trying to call the `querySelectorAll` method on an `undefined` value.

To solve the error, run the JS script after the DOM elements are available and
make sure you only call the method on valid DOM elements.

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Cannot read property 'querySelectorAll' of Undefined in JS

Cannot read property 'querySelectorAll' of Undefined #

Conclusion #

Borislav Hadzhiev

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Cannot read property 'querySelectorAll' of Undefined in JS